I noticed my battery voltage had been looking a little on the low side several months ago. Checking individual batteries with a volt meter I originally found 2 dead batteries in the string. So I simply removed those two batteries from the bank and reduced my energy consumption substancially as well. Fast forward 2 months and I found myself in the same predicament with 2 more dead batteries in the bank. Again, I removed them from the bank and left one 155 ah 2 battery connection in place, and turned the inverter off. One week later after I checked on the set up and with minimal load at night (13 watt led light on) the voltage dropped low enough to trigger the lvd on the inverter and it shut down. So now all 6 of my batteries are junk.

My charge controller display averages 50-60 volts at 10 amps input from my array during the day so I feel comfortable that my system is working properly. And I also understand that I have effectively achieved over 1200 cycles from my battery bank.

My questions are: Does my setup look correct as in does anything stand out as a battery killer?

Am I correct that agm batteries do not have the cycle life as traditional flooded batteries?

Comments

3.5 years from a UPS rated battery (rated more for float charging and using when a power failure occurses)is probably not that bad, especially if they have been kept on the warm side (75F or warmer).

I always like to start with your loads and design a "balanced system" that will give you good life and meet your expectations.

So, lets say your needs are 1 kWH per day. That is not a large load (generally lighting, cell phone charging, RV water pump, Laptop Computer for a cabin). The typical "optimum" battery bank would be 2 days of storage (bad weather/no sun) and 50% maximum discharge. I am going to start this as a 12 volt battery bank design, because I think your are better off with with a low power/high efficiency 12 volt 300 Watt AC inverter vs a larger inverter (which uses more power just to turn on... Typically 6 Watts for a smaller inverter, closer to 10-20 Watts just to run your larger inverter). 24 volt would be OK too if you need a larger inverter for short/heavy loads (Microwave, power tools, etc.)...

Next, sizing the solar array. Two sets of calculations, the first based on the size of the battery bank. Larger battery banks, need more charging current. 5% rate of charge is good for weekend/summer usage. 10%-13%+ is better for full time off grid.

This is an issue with "too large" of battery bank. You need a large solar array for optimum (10%) rate of charge. In the olden days, people where told to add batteries to increase the output of their system. Back then batteries where cheap and solar panels were expensive. It is now solar panels that are cheap and batteries that are expensive... And the better way to "up size" an off grid solar system is to add solar panels (which are actually generating electricity, not adding to storage).

So, your 600 Watt array is just about the smallest you would want (based on load). If your AC inverter drew 15 watts "just being turned on" and was left running 24 hours per day (15w*24h=) is 360 WH per day just for the inverter...

Anyway, using a MorningStar 300 Watt PSW AC inverter would draw 6 watts, and if you use the auto sleep mode (inverter sleeps unless there is >6 Watt AC load), and you would use a fraction of this.

Lots of guesswork here. Energy usage is a highly personal set of choices. For example, if you need a minimum of 1,000 WH per day in winter and don't want to use a genset, you might want 2x larger solar array (1,244 Watt array) (better than adding batteries).

So, here I am suggesting that you go with a lower voltage battery bank to get a "more optimal" AC inverter for your smaller loads. But, the problem is that you need a much larger MPPT solar charger (and more expensive) to output that much current into a 12 volt bank:

600w of panels likely produces ~450w in decent sun, which should cover your 1kwh/day load (if no shading issues), but may be a bit low for catching up after a run of gloomy weather. Do you have a generator or other means of charging?

At 50% SOC, a 450ah bank is down 450x24x0.5=5400wh. In winter, average daily insolation (used Nashville) is ~3hrs, so the array might produce 1500wh/day on average. With a 1000wh load, that only leaves ~500wh to catch up with the 5400wh deficit. Absent a generator to help, this could leave the bank in a low SOC for longer than it really should.

For a full-time off-grid system, something on the order of a 10% rate of charge capacity is reasonable, so for a 10800wh bank, that would be 1080 ÷.75(panel derate) = 1440w array.

For a weekend cabin type use, where the pv can catch up with no loads, 600w could work ok, but for more full-time I'd want generator to supplement charging and/or more solar.

A reasonable life expectancy for T105s would be ~5yrs, but would be much shorter with problems like chronic undercharging or leaving at <70% SOC for many days at a time.

Wow! Thank you so much for the information packed reply! I will have to read it several times to absorb it all!

I now must add that I do in fact have 2 additional 100 watt panels in storage that i can add to my array that will bring it to 800w.

My charge controller is an mppt controller. the maximum allowable input is 20 amps and 100 volts. I think i conveyed that incorrectly originally. I purchased the 2 extra panels as a "just in case" measure for future upgrades which I now see I will need to utilize.

I just looked up the specs on my 24v inverter.

no load current draw is .65 amps and power saving mode is .05 amps.

So if I'm calculating correctly that would bring my daily consumption from the inverter to 15.6 amps times 24 volts which brings me to 374 watt hours per day!! wow!

You are spot on with my loads being led lighting, an rv water pump, cell phone and laptop charging, etc. My only other major load is a converted deep freezer which I've averaged with a kill-a-watt meter to consume 250 watt hours per day. My on demand water pump pulls 120 watts while running and the freezer pulls 110 watts with a good spike upon start up. I sized my inverter for the use of the extremely occasional power tool use and 1000 watt microwave use.

I'm a little foggy about the down sizing of my bank voltage to 12 volt. I was under the impression that the higher bank voltage was more efficient. I do understand i could effectively get my needed ah with less batteries at 12 volts but at the trade off of having to purchase another extremely expensive inverter.

Having read your reply a few more times I understand further what you are saying about down sizing my inverter for more efficiency.

I will have to re read my manual for my charge controller, but i seem to remember that the capabilities were cut basically in half when used on a 12 volt setup vs. 24 volt and I think that is why I ultimately decided on 24 volt when I built the system.

Again, thank you so much for your insight! I cant wait for more info!!!!!

With fridge and pump loads, I'd stay with 24v. The 300w 12v inverter Bill mentioned is nice (I use one), but won't likely start the fridge, and maybe not the pump. With your existing inverter you can likely save a fair bit by unplugging chargers etc at night so it can use the search/power saver mode.

To use the extra pair of panels, you'll need get a second charge controller.

You might consider a single string bank for [email protected] = 5400wh. Using 1000wh/day and min 50% that gives you a couple days no sun. Use the cash saved on the second string towards a small inverter generator?

thanks estragon! I do in fact have a standby generator and an automatic 24v charger for "just in case"

As far as adding the extra charge controller. I was under the impression that adding the 2 additional panels would still keep me under the 100 volt, 20 amp threshhold that my charge controller is rated for. I would have to re- crunch those numbers but i seem to remember the tech support at renogy telling me that in 24 volt mode the charge controller would accept 800 watts of pv.

I will also mention that this cabin is in fact mostly a weekend cabin. since it is vacant all day and only utilized for a couple hours at night maybe 3 days a week. The only constant draw is the freezer and the inverter itself.

Another question... My inverter does in fact have a power save mode. Would it be harmful to put it in power save mode constantly? That way it would only come on when the freezer kicks on?

A 20a controller might "accept" 800w of panel, in the sense that it can be overpanelled to that extent. 800w ÷24v charging = 33a, or ~50% more than the controller rating. Realistically, 800w rated pv could put out 600w (25a) or so in "normal" operating conditions, but the controller is limited as a 20a load, so the last potential 5a won't be drawn by the controller. Depending on ambient temps etc, the controller may cut current drawn further as it heats up.

We sometimes overpanel because pv is pretty cheap, and overpanelling lets us get some useful charge in overcast conditions. There's a limit to how far you can go with that though, which is where the 800w "accept" number comes in.

In your case, you're underpanelled, so you want all the amps you can get. Adding a second 20a controller would split the 800w into two 400w arrays, with each outputting ~400x.75÷24= ~12a each. If a cloud goes by and there's more power to be had, you get it.

Putting the inverter into search mode shouldn't normally be a problem, although a couple of things to watch for.

A microwave led etc that doesn't draw enough power to bring the inverter out of search mode, but gets enough power from the search probe to light will flash at the search probe rate (typically a few seconds). Best to unplug or switch off such things. At best, they're annoying.

The other is things like fridge electronics, which can sometimes get messed up with stuff like defrost cycles. I have a simple fridge that does fine with inverter search, but some of the more complicated ones can have issues.

great insight!!! everything is normally unplugged accept for the freezer.. which is a standard deep freezer with the thermostat played with to run fridge temps. I'm going to play with some numbers tonight. Thank you both for your help so far.Please keep it coming!!!!

I'd suggest checking the voltage label of the panels and when you add panels, split them into 2 strings of 4 - IF the Voc of 4 in series, is less than 100Vdc You use the Voc for your voltage limit, and generally add 20% of the voltage for cold weather, since cold panels produce higher voltage.

I concur with keeping the system at 24V, the fridge needs the benefit of a higher voltage on the DC side of the inverter. Try looking for a high effiecency inverter in the 800 -1200W range - you need some good surge capability for starting the AC motors.

If you can run your inverter in search mode/power save with the freezer/refrigerator--Then you should be fine. Just run the inverter full on when you are there.

If you are running the freezer/fridge while you are not there, then having a larger battery bank and solar array is a good thing. More or less, the weakness of off grid solar power systems is that if you do not have enough sun (bad weather) or something else goes wrong (somebody forgot to turn everything off when you left, something "broke", etc.), the battery bank is the major item "at risk".

Lead acid batteries (and most rechargeable batteries) do not like to be taken to zero volts (and hate "reverse charging"). Winter weather and unattended loads are frequently good ways of "murdering" your battery bank (and losing the food in the fridge).

Lots of great insight here again! Thank you all so much! If I remember correctly I do think I would have had to wire 2 strings of 4 panels in parallel to utilize the full 800 watt capacity of my charge controller. I just hot home for the evening so I'm going to dig up my paper work. I must say I'm absolutely blown away by the wealth of knowledge you fine folks are flooding me with!. And I honestly feel lucky to have been living off grid for this long without a hiccup until now! Looking forward to conversing more!

Considering the cost of a new set of batteries you really ought to consider the minor additional cost of a 40 - 45 amp MPPT charge controller. Your controller is actually limited to 20 amps, OUTPUT. You need higher current to keep your battery bank happy and healthy.

i believe the collective suggestion is definitely a charge controller upgrade, along with additional panels which fortunately I have 2 of on stash. What do you, fine professionals, suggest as far as brand?

As far as the battery bank is concerned... Am I still making a sound decision by purchasing 8 trojan t-105s for a 450 ah 24v bank?

If trojans are available locally at a reasonable price, they'd be a good choice IMHO. Decent T105 (GC sized) batteries are widely avaikable though, so if the trojans are a sunstantial premium or need to be shipped in, I'd consider going with a local alternative.

In general, GC type batteries can work well. Being flooded, you can (and should) monitor SG, which is much better than voltage alone for catching impending problems.

The Renogy controllers seem to work for those using them, and there's a lot to be said for going with a controller brand you're already familiar with.

Good morning, experts! Did a bunch of spec reading and number crunching last night! To my pleasant surprise, my current charge controller is in fact a 40 amp and not a 20 amp charger!!!! I apparently forgot that over the course of 3 years. Lucky me! I plan to keep my current mppt charge controller in place, add 2 more 100 watt panels to my array.. 2 parallel strings of 4 panels each, bringing my voc to 90 volts max and 800 watts, and I will also leave my inverter in power save mode while I'm away. The Trojan t-105 batteries are available to me locally for 125 dollars a piece. I'm going to call the place today and see if their are any alternatives in stock. I like the idea of using the Trojans based on their reputation. I also have a wholesale account through interstate batteries. Their gc2 equivalent is 129 my cost a piece.. but I noticed they offer a 2300 series golf cart battery that boasts 240 ah for 10 dollars more. That would bring my bank up to 480 ah. Am I pushing the limits bringing my ah up any higher than my original 465 ah? I'm a little concerned with dropping down to 450 ah from 465ah. I know it's only 15 ah but that's 365 wh!!! That's basically what my inverter was consuming in a day. What do you, fine folks think?

Local Trojans at $125 sounds pretty reasonable to me. IIRC, some Trojans want a bit higher charging voltages than other flooded, but this isn't likely an issue for your application. It's mainly if the bank is cool/cold and/or the controller isn't adjustable.

Whichever you decide on, I'd check voltage in case they've sat around without a periodic top-up charge. A good dealer shouldn't let that happen, but...

A <10% capacity difference isn't really material, as identical batteries could vary by about that anyway.

Note that 4x Vmp~18 volt panels is getting a bit high for Voc-cold... If you have cool winters, Voc-array-cold may exceed the 100 vdc maximum input voltage for your controller (very hard freezes could get upwards of 108 VDC)...

Note that 4x Vmp~18 volt panels is getting a bit high for Voc-cold... If you have cool winters, Voc-array-cold may exceed the 100 vdc maximum input voltage for your controller (very hard freezes could get upwards of 108 VDC)...

-Bill

my panels are actually 12 volt panels. The voc listed on them says 22.5 volts. Is it possible that on freezing days the panels could go over their rated open circuit voltage?

A string of 4 would be ~90v, but that's at ~room temp. In freezing temps, that may be getting too close for comfort on a 100v max controller.

What happens is on a cold morning (lows are often right around dawn) the voltage rises in the weak morning light, and peaks before the sun hits the panels to warm them. Even with little current in the weak light, the controller can be damaged with the overvoltage, and many controllers will log the event to void warranty.

To check, find the record low ever recorded at your location, and the voltage-temperature adjustment coefficient for your panels. If too high, a smaller string size or higher voltage controller will be needed.

Panels run "hot" when under direct sunlight. However, first thing in the morning, freezing temps, and clear sky, the cells will start out at ambient temperature (or even a bit lower) until they get heated by the sun.

The switching transistors in the charge controller only care about "overvoltage", even if the there is hardly even any direct sun on the panels.

Using the Midnite tool (kind of "busy" for answering this question).... Voc=22.5 volts, 4x in series, 10 degrees F, get 101.2 Volts calculated for Voc-array-10F operation.

Yes with 100 watt panels it is. You have to either do 2 in series and 5 in parallel with a combiner box and or fuses. Or buy 1 panel and do 3s 3p again with a combiner. Imo 100 watt panels and a 100 volt cc works better with smaller like rv 12 volt 450 amp hour 600 watt systems. For 1000 watt arrays 4 250 watt panels work best with 100 volt cc.Trogen recommend normal 10 to 13% 20% max charge rate. 45 amps at 10% with your 450 @24 bank up to 90 amps max. As estragon said your way under paneled. 450×29÷.77×.10=1694 watts at 10% charge rate.